In connection with optical communications, optical signal processing, or optical computing, it is often necessary to rapidly modulate the intensity of an optical signal, such as a laser beam, by some electronic means. In the case of laser beams derived from semiconductor devices, this function is conveniently performed by means of heterostructure laser modulator. Heterostructure laser modulators possess the advantages of small size, robustness and long-term reliability.
In heterostructure laser modulators as known in the art, the internal structure of the modulator is based upon a series of stacked, wide and thin layers of differing semiconductor materials disposed on a semiconductor substrate. Such a stacked assembly is known generally as a "heterostructure", and it is fabricated by epitaxial semiconductor material growth techniques familiar to those acquainted with the state of the art. The resulting heterostructure laser modulator requires that the carrier input and output laser beams be shaped into a wide, yet thin cross-sectional beam to accommodate the semiconductor layer structure comprising the modulator. This requirement for beam shaping constitutes a serious disadvantage for conventional heterostructure laser modulators in terms of complexity and cost. In addition, it is required that the laser beam be carefully tailored to contain large and specific levels of astigmatism, which is detrimental to the reliability and ruggedness of the laser modulator.
To operate, the heterostructure requires a biasing potential across this stack. Previous heterostructure modulators do this by a pair of striplines, one connected electrically to the topmost layer of the heterostructure, the other to the bottommost (bottommost being the active layer of the heterostructure disposed most closely to the substrate). Because of the stacked arrangement of the heterostructure layers, there is no ready access to the bottommost layer by which one can effect electrical connection. Previous laser modulators gained access by running one of the striplines along the substrate face that was opposite to the face on which the heterostructure resided, and forming a path through the substrate to the heterostructure. However, by running the two biasing striplines along faces so widely separated from one another, one introduces considerable inductance into the modulator circuit, which effectively precludes operation of the modulator at microwave frequencies, or higher.